The invention described herein was made by an employee of the United States government, and contractors of the United States government, and may be manufactured and used by or for the government for government purposes without payment of any royalties thereon or therefore.
The present invention relates to a hollow cathode electron source, which finds application in plasma generation, and more particularly to a long-life hollow cathode plasma generator and to its design, manufacturing process and assembly. These manufacturing processes include contamination control procedures which cover hollow cathode component cleaning procedures, gas feed system designs and specifications, and hollow cathode activation and operating procedures.
Cathodes emit electrons when elevated in temperature by a process known as thermionic emission. Thermionic emitters generally consist of a wire that is made of some refractory metal, which may be typically made of tungsten or molybdenum. The wire is then coated, or impregnated with some low work function material, such as barium carbonate, and subsequently ohmically heated.
Hollow cathodes have been in existence for over ten years. Hollow cathodes have been developed to an advanced state of technology readiness for ion propulsion. Ionic propulsion may be defined as propulsion by the reactive thrust of a high-speed beam of similarly charged ions ejected by an ion engine. In ground tests they have demonstrated high emission currents of greater than 30 amperes, and long lifetimes, with modest power requirements of less than 100 watts. Hollow cathode plasma sources have demonstrated versatile and effective operation as plasma contactors in ground testing of various devices. This testing includes plasma bridge neutralizers for ion thrusters, plasma contactor demonstration experiments for the electrodynamic tether, and space station structure potential control experiments.
Hollow cathodes have also been flown in space as components of ion propulsion systems and spacecraft charging/charge-control systems, including ATS-6, SERT-II, SCATHA, and SCSR-1 flight experiments. Demonstrated capabilities in space tests include lifetimes of 10,000 hours and more than 300 restarts. NASA flight experiments have demonstrated hollow cathode plasma contactors to be effective in controlling both the negative charging and differential charging of the spacecraft frame. Hollow cathodes have been operated in space under a variety of orbital and environmental conditions; on spacecraft, including an Agena vehicle, on communication satellites, and on the space shuttle. Environments include those of low-earth orbits, sun-synchronous high inclination orbits, and geosynchronous orbits.
All of the above hollow cathode development was accomplished with mercury as the hollow cathode expellant, or xe2x80x9cworking fluid.xe2x80x9d For a variety of reasons, which includes spacecraft contamination, the present hollow cathodes preferentially use an inert gas, such as xenon, as the expellant. Subsequent to the transition from the use of mercury to xenon in the early 1980""s, there have been, and continue to be, failures of hollow cathodes in the United States, in Europe, and in Japan. These have impacted both research and development activities and flight programs. The failures have apparently been primarily due to inadequate procedures and protocols to control contamination during the fabrication, assembly, testing, storage, handling, and operation of the cathodes; as well as, inadequate design and process features. To date, the only successful extended duration tests, that have been reported of using inert-gas hollow cathodes at high emission currents of greater than 1 ampere, have been conducted, by the NASA Lewis Research Center. These successful extended duration tests were implemented by the use of the design features and processes that are further described herein.
U.S. Pat. No. 3,944,873, granted Mar. 16, 1976, to J. Franks, et al., discloses a hollow cathode of cylindrical shape. A cathode encloses an anode having a pair of screen electrodes, symmetrically disposed about and parallel to the plane of the anode. The anode has a central aperture and another aperture may be made in the cathode diametrically opposite the first aperture.
U.S. Pat. No. 4,049,989, granted Sep. 20, 1977, to R. H. Bullis, et al., discloses ion production using a permeable electrode having apertures and a central electrode. A wire mesh grid is placed symmetrically about the permeable electrode.
U.S. Pat. No. 4,087,721, granted May 2, 1978, to G. Mourier, discloses an ion source that is comprised of a hollow cathode discharge arrangement having an anode placed between two cathodes. The cathode has holes through which some of the ions of the plasma escape.
U.S. Pat. No. 4,377,773, granted Mar. 22, 1983. to A. Hershcovitch. et al., discloses an ion source that is comprised of a hollow cathode and an anode base having electrically connected anode covers.
U.S. Pat. No. 4,428,901, granted Jan. 31, 1984, to W. H. Bennett discloses a hollow cathode, that is held inside of a cathode holder, as well as, a hollow anode that is supported by a conducting support. A diode envelope surrounds the hollow cathode.
U.S. Pat. No. 4,894,546, granted Jan. 16, 1990, to R. Fukui et al., discloses a cylindrical hollow cathode having upper and lower circular anodes that are placed at the two ends of the cylindrical cathode, where each of the anodes have circular openings.
U.S. Pat. No. 5,075,594, granted Dec. 24, 1991, to R. W. Schumacher, et al., discloses a hollow cathode used for discharging ionized plasma of an ambient gas, such as xenon. A flat anode extends perpendicular to, and is intersected by, the axis of the cathode. A keeper/baffle electrode, which may also be a plate, is disposed between the cathode and anode. Even though this device is a low impedance device, it will not yield electron emission currents to an external electrode in the multi-ampere range, within a voltage range of 20 volts.
U.S. Pat. No. 5,241,243, granted Aug. 31, 1993, to G. Cirri, discloses a plasma generator that is comprised of a hollow cylindrical cathode and one or more anodes.
U.S. Pat. No. 5,352,954, granted Oct. 4, 1994, to G. Cirri, discloses a plasma generator that is comprised of a hollow cylindrical cathode and one or more anodes having holes.
U.S. Pat. No. 5,569,976, granted Oct. 29, 1996, to N. V. Gavrilov, et al., discloses of an ion emitter that is comprised of a hollow cathode at one end and a coaxial rod-shaped anode at the other end. The hollow cathode encloses the rod shaped anode.
U.S. Pat. No. 5,581,155, granted Dec. 3, 1996, to A. I. Morozov, et al., discloses a plasma accelerator that is comprised of a hollow cathode and an annular anode.
All of the above referenced prior art relate to high voltage acceleration systems. Further, they do not teach of a self-regulating emission control system. None of the prior art relates to an ionic emission apparatus, having low current capability, with the exception of U.S. Pat. Nos. 5,075,594 and 4,428,901, which disclose the use of electron emission apparatus. Only U.S. Pat. No. 5,075,594, teaches of a low output impedance, whereas all of the others have an undesirable high output impedance, that is not suitable for use in space station applications.
In addition, none of the above referenced prior art provide an attained performance reliability having a demonstrated lifetime in excess of the present state-of-the-art 500 hours, when operated at emission currents of approximately 1 ampere.
These devices in the past have exhibited unstable operating characteristics and shortened lifetimes as a result of design and processing problems. Until the initiation of the present program, there have been no inert gas hollow cathodes that had demonstrated lifetimes greater than 500 hours, when operated at emission currents greater than 1 ampere.
The present invention differs from the aforementioned prior art inasmuch that the approach is not limited solely by the design of the apparatus but also includes the method of manufacturing processes and procedure in order to obtain a highly reliable and repeatable design commensurate with a high life expectancy. The advancements demonstrated in the manufacturing processing include contamination control procedures which cover hollow cathode component cleaning procedures, gas feed system designs and specifications, and hollow cathode activation and operating procedures.
Accordingly, it is therefore an object of the present invention to provide an electron emissive hollow cathode apparatus that will provide reliable, stable and repeatable operation over a lifetime that is in excess of 17,500 hours.
It is an object of the present invention to provide an electron emissive hollow cathode apparatus that will provide reliable, stable and repeatable operation over a broad range of operating emission currents of at least a 6:1 ratio.
It is still another object of the present invention to provide an electron emissive hollow cathode apparatus that will provide reliable, stable and repeatable operation, while permitting electron emission currents of up to 30 amperes emission to an external anode, at voltages of less than 20 volts DC.
It is a final object of the present invention to provide a method of manufacturing an electron emissive hollow cathode apparatus that when adhered to, will provide reliable, stable and repeatable operation over an expected lifetime that is in excess of 17,500 hours.
These as well as other objects and advantages of the present invention will be better appreciated and understood upon reading the following detailed description of the presently preferred embodiment taking in conjunction with the accompanying drawings.
The present invention relates to the design and processes that are required to fabricate long lived hollow cathode assemblies, which will exhibit stable and repeatable operating parameters.
These processes have been demonstrated for emission currents up to 24 amperes and lifetimes greater than 10,000 hours, and they have been incorporated in a cathode design which is proposed for controlling the floating potential of International Space Station Alpha (ISSA). These design and processes permit stable and repeatable operation over a broad range of emission currents under variable and uncertain current demand, at electron emission currents up to 30 amperes at potentials of less than or equal to 20 volts, and with a life expectancy of at least 17,500 hours.
The International Space Station Alpha (ISSA) power system is designed with high voltage solar arrays which operate at output voltages of typically 140-160 volts, and is configured with a xe2x80x9cnegative groundxe2x80x9d that electrically ties the habitat modules, structure, and radiators to the negative tap of the solar arrays. This electrical configuration and the plasma current balance that results will cause the habitat modules, structure, and radiators to float to voltages as large as 120 volts with respect to the ambient space plasma. As a result of these large negative floating potentials, there exists the potential for deleterious interactions of ISSA with the space plasma. These interactions may include arcing through insulating surfaces and sputtering of conductive surfaces due to the acceleration of ions by the spacecraft plasma sheath. Both of these processes may result in changes in the surface material properties, in destruction of coatings and in contamination of the surfaces due to redeposition.
The space experiment SAMPIE (for Solar Array Module Plasma Interactions Experiment) was recently flown on the Space Transportation System STS-62 and provided valuable validation of the theoretical models of spacecraft charging that were used to predict the station floating potentials. The flight data that was acquired from this experiment, which quantified the current collection to station solar array elements, confirmed the need for a plasma contactor to control the potential of the space station.
A decision was made, that was based on its potential effectiveness, to baseline a plasma contactor system on ISSA as the solution to alleviate plasma interactions. Consequently, NASA initiated a plasma contactor development program as a portion of the ISSA electrical power system.
There are several major derived operational requirements for the station plasma contactor system which include: (a) the capability to control station potential to within 20 volts of space plasma potential; (b) emit electron currents up to 30 amperes under dynamic and variable conditions; (c) operate for up to 17,500 hours without degradation; (d) minimize consumables; (e) be single-fault tolerant in design; (f) be compatible with all space station utilities; (g) be robotically serviceable; and (h) incorporate health monitoring instrumentation, including instrumentation to measure the plasma return current.
For the ISSA application, efficient and rapid emission of high electron currents is required by the plasma contactor system under conditions of variable and uncertain current demand. A hollow cathode assembly is well suited for this application and was therefore selected as the criteria for the design of the station plasma contactor system.